Mycoplasma fermentans, a mycoplasma species that colonizes humans, was first described in 1953. It has been detected in and isolated from tissues of AIDS patients and appears to be associated with a fatal necrotic disease. In order to satisfy Koch's postulates for demonstration of pathogenicity, we have intraperitoneally inoculated six groups of 4 rhesus macaque monkeys each with one of the following 3T3 tissue cell culture-grown preparations: the original M. fermentans isolate from an AIDS patient, strain incognitum; reference fermentans strain, PG18; a synovial fermentans isolate, strain KL-4; a tissue-culture dependent strain, IM-1; a heterologous mycoplasmal synovial isolate, M. hominis strain 1620; or just 3T3 cells alone. The monkeys were inoculated November 1, 1991. The four monkeys from each group have had their temperatures taken and been weighed weekly. Blood is drawn every month. At this date, at least one monkey from each fermentans group has evinced either a weight loss or an inability to maintain weight. Nevertheless, no serum antibodies have been detected in any of the 16 monkeys that received M. fermentans. In contrast, the 4 monkeys inoculated with M. hominis have demonstrated substantial rises in serum antibodies against hominis proteins. Cytokine responses are currently being measured. These results are similar to those observed during the initial M. fermentans study in animals performed by Dr. Shyh-Ching Lo (WRAIR) as well as an on-going SIV study in rhesus monkeys at the University of Wisconsin.